The similarity of polynucleotide phosphorylase from M. luteus and E. coli has been confirmed by structural studies which show that the primer-independent M. luteus enzyme is also a three-subunit enzyme. Conversion to primer dependence by trypsin treatment is accompanied by the removal of a carboxyl terminal peptide without large conformational changes in the protein. Regulation of cyclic nuecleotide phosphodiesterase by calmodulin has been shown to be only one example of several similar calmodulin regulated systems. Calmodulin forms equimolar complexes with several polypeptides such as the catalytic subunit of phosphodiesterase, the large subunit of its inhibitory protein, myosin light chain kinase and troponin I. Since these polypeptides are all different they may share a common calmodulin binding domain. The calmodulin binding domain of phosphodiesterase plays a regulatory role by itself. Removal of this domain by limited proteolysis results not only in the loss of calcium regulation but also in a large increase in enzyme activity. It therefore behaves as an inhibitory subunit. Another regulatory mechanism shared by several calmodulin binding proteins is their ability to be regulated by cAMP dependent phosphorylation. The activity of mysin light chain kinase and its affinity for calmodulin was shown to be affected by phosphorylation.